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   from the issue of November 1, 2007

13-year tree study challenges growth, reproduction tradeoff


A long-held assumption among biologists is that plants face a tradeoff between growth and reproduction. If they grow a lot, they have few resources left for reproduction, and vice versa.

But new research based on a 13-year study indicates that's not necessarily the case, at least for some species of oak trees. In a paper published in the Oct. 15-19 online version of the Proceedings of the National Academy of Sciences, a team led by Johannes M. "Jean" Knops of UNL showed that growth and reproduction are both controlled by rainfall and competition for resources between growth and reproduction is negligible.

In the study conducted at the University of California's Hastings Natural History Reservation in Monterey County, Calif., Knops and his colleagues, Walter Koenig of the Hastings Reservation and William Carmen of Mill Valley, Calif., collected growth and reproduction data on 239 mature oak trees from five species intermingled in the same geographic area from 1994 through 2006. They used devices called dendrometers fitted around the main trunks of the trees to measure growth and counted acorns to measure reproduction.

Three of the species that Knops and his colleagues studied (valley oak, blue oak and coast live oak) are pollinated and produce acorns in the same year. The other two species (canyon live oak and California black oak) have two-year reproduction cycles, producing acorns a year after pollination. The two-year species enabled them to determine that growth and reproduction don't have a direct negative correlation.

As expected, the scientists found a negative correlation between growth and reproduction with the one-year species. But in species where growth and reproduction occur in the same year, it's impossible to determine if there is a tradeoff between the two factors.

With the two-year species Knops and his colleagues found a positive correlation between growth and reproduction in the same year (i.e., more growth, more acorns, and vice versa). But they found a negative correlation (i.e., more growth, fewer acorns, and vice versa) between growth in the year of pollination and reproduction the following year, when acorns were produced.

They concluded that rainfall controlled growth and reproduction independently of each other.

"In life history studies, one of the key assumptions is that there's a tradeoff between growth and reproduction, which most studies pretty much take for granted," Knops said. "When we looked at it a little more precisely, it was pretty clear that both growth and reproduction are controlled by rainfall, but the negative correlation between growth and reproduction isn't causal. It's just an artifact of all these weather variables."

Knops said it's not surprising that there isn't a direct correlation between growth and reproduction. Still, he said, the trees can't produce an unlimited number of acorns and the study showed strong negative correlations in annual acorn crops, so something must be controlling that. Squirrels and other herbivores are the prime suspects.

"These trees live hundreds of years and they make acorns to produce new trees, not to get fat squirrels." Knops said. "So if they produce regular seed crops, you get a buildup of squirrel population. But if they fluctuate the crop, there are some years where they're loaded with acorns and squirrels can't eat them all, and in other years there are no seeds, and the squirrel population goes down.

"It looks like the evolution of this acorn productivity pattern is not directly controlled by a tradeoff within the tree for resources, but more this evolutionary game with animals that eat the acorns. It's a much more complicated, much more interesting story that way."



13-year tree study challenges growth, reproduction tradeoff
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